The present invention relates to a magnetic field protecting and screening multi-layer textile construction.
The present invention may be generally applied to all electronic devices, either of a small or large series, including at least an audio function, that is a (voice or music) sound emission from loudspeakers or the like devices, or a sound reception by many types of microphones, which general application field comprises a number of device families and sub-families such as:
Telephonic field:                Cellular phones        Land-based telephonic articles (phones, hand-free phones and related fittings) and Skype/SAT phones.        
Communications:                Walkie-talkies        Helmet and protective caps built-in audio devices        Professional radio apparatus for military, safety, civil protection and outdoor works.        
Entertainment:                Hand-held Hi-Fi devices (MP3 players, earphones, headphones and hand-held acoustic boxes)        Professional audio fittings (microphones, headphones, loudspeaker components)        TVs (LCD displays, monitors, hand-held DVD players).        
Transports:                Satellite navigator systems including voice directions        Automotive (Car Hi-Fi, hands-free warning and voice equipments)        On board communications devices (trains/airplanes/ships).        
Other applications:                Computers (monitor speakers, external acoustical boxes, auxiliary microphones, webcams)        Domestic application fittings (intercommunication or interphone systems, in-home audio communications)        Acoustical devices for ear impaired persons and other medical devices.        
In many hand-held systems, because of designing and size miniaturizing reasons, loudspeakers and microphones are frequently arranged, as it is well known, near antennas or SIM cards.
In this case, if the stationary or low frequency magnetic field generated by the above acoustic components exceeds a given or target threshold value, then it may cause a demagnetization of the SIM card or other electromagnetic interferences with antennas, which cannot properly operate.
Moreover, the acoustic component generated magnetic field draws or attracts contaminating metal parts from the environment causing them to pass through component protecting screening or shielding units, thereby the overall equipment cannot properly operate because of a mechanical interference phenomenon.
Thus, in the above systems, it is necessary to reduce the acoustic component generated magnetic field strength, and improve said component protecting assemblies while reducing other electromagnetic interferences with other electronic components nearby.
Thus, the aim of the invention is just to solve the above problems affecting internal acoustic components, such as loudspeakers and microphones, which are very delicate components which must be further protected against intrusion of water and solid particles (powder, dust, dangerous fragments) by a protecting system which does not negatively affect the target emission and reception sound features.
The above, in turn, results in very complex functional requirements of the acoustic component, since it is necessary to combine good sound transmission characteristics, to be achieved by openings formed through outer shell assemblies, with a satisfactory component protection (which, in turn, requires to insulate as far as possible the acoustic components from the outer environment) with a proper magnetic field screening (which, per se, requires to insulate as far as possible from the outer environment the acoustical component).
In standard conditions, conventional methods for achieving the above aim and objects comprise applying porous protective devices on outer openings and, if a target protection level or a full magnetic field screening is required, said outer openings will have a very complex design.
In a typical cellular phone, as shown by the reference number 1 in FIG. 1, said outer openings comprise three openings at the main loudspeaker 2, the microphone 3 and the hands-free/ringing loudspeaker 4.
To properly protect acoustical components, many protective devices are at present used, depending on design requirements, the protective type and degree, and on whether or not a full screening from the magnetic field is required.
Hereinbelow is shown a list of main possible approaches, properly arranged in an increasing protection level order.
2.1. No protection. The acoustical component is exposed to the outer environment (this being an uncommon solution).
2.2. Molded plastic material protecting bars or grids, with an anti-impact function only.
2.3. Large mesh opening protective nets, either made of a metal material (for example in spherically protecting microphones) or a molded plastics material, and operating as anti-intrusion elements for small articles (such as pencils and the like).
2.4. A non-woven material screen or shield, with an optional water-repellent or hydrophobic treatment, arranged on the acoustical component front portion.
2.5. A synthetic single-thread or monofilament technical fabric material, with an optional water-repellent treatment.
2.6. A water-repellent membrane, made of an E-PTFE (expanded PTFE) material.
If, in addition to providing protection against polluting particles and liquids, the acoustic component generated magnetic field strength is to be reduced, then the protective construction design becomes very complex and, accordingly, to each of the above solutions 2.1-2.6 is usually added:
2.7. A small perforated plate made of a ferromagnetic material and having such a size and design as to provide a target screening efficiency.
Said component being arranged between the speaker and a filter having a very fine porosity or, alternatively, between two porosity filters.
The first three solutions do not provide protection against liquid materials, and have only a small efficiency against intermediate-large size solid articles (see the above 2.2. and 2.3. items).
On the contrary, the above 2.4. to 2.6. solutions provide a good protection even against a possible intrusion of contaminating liquids and powder into the acoustic component.
Each of the above standard solutions, in combination with 2.7., is adapted to combine a required protection against liquid and solid materials, depending on the target solution efficiency, with a magnetic field screening.
An overlapping of multiple layers of a protecting/screening material tends to worsen the component acoustic performance, since they represent additional obstacles to a normal air flow.
An optimum solution would be that of designing protecting/screening means having a low acoustic impedance and, if possible, providing a trade-off between the required protecting/screening level and the related acoustic impedance.
In the most common cases, such as in cellular phones, the screens are assembled together with synthetic foamed material gaskets and bi-adhesive strip templates, to provide a strong adhesion of the screen to the apparatus outer body.
It should be apparent that, if multiple layers are provided for protecting/screening acoustic components, then the additional elements to be assembled (gaskets/adhesive tapes) and their assembling steps and the resulting overall thickness will greatly increase depending on the protecting/screening layer number.
FIG. 2 shows some examples of the above components, made of a monofilament polyester (2.5.) technical fabric material, and further including an annular gasket with an adhesive area to be glued on a cellular phone shell.
From an acoustic standpoint, such an optional protective screen must not change the inlet or outlet sound flow in comparison with a designed target one.
Usually, for a large number of consumer acoustic products, it is necessary to reduce to a minimum the sound pressure level attenuation.
Accordingly, the protective screen should be “acoustically transparent or clear” and provide its protecting function with an interference as small as possible with the acoustic component inlet or outlet sound flow.
This is very common in a cellular phone protecting screen, which should not excessively attenuate the speaker sound or microphone sensitivity, thereby allowing to use small, light and inexpensive acoustic members.
On the contrary, in other cases, for example in a middle-high range acoustic product, it is desirable that the protecting screen provides a true acoustic function, to roll-off possible emission peaks, or deformed sounds, to in turn differently balance or compensate for the acoustic component frequency response.
In any case, the textile material component, either of a woven fabric or a non-woven or membrane type, should have the designed acoustic features which may range, depending on the application, from a maximum “acoustic transparency” to a given sound attenuating level.
To quantify the above acoustical characteristics, different evaluating methods can be used.                The Standard Test Method for Airflow Resistance of Acoustical Materials (ASTM C522-87) correlates the air flow-rate and the load loss in a case of a stationary air flow passing through the textile product. The results are given in Rayls MKS, and low values of this parameter correspond to “acoustically transparent” materials.        The “acoustic impedance” value is based on the same parameters as above, but it is measured in an air flow alternating regimen, that is under conditions closely corresponding to the acoustic application environment.        Finally, if it is possible to directly test the acoustic screen in a final configuration thereof (that is with its final shape and size identical to that installed in a commercial end-product) then a direct measurement of the sound pressure level may be carried out, either with or without a textile screen arranged between the sound source and the measurement microphone.        
The result is usually provided in decibels, dB(SPL), and is referred to different forming methods (ISO/FDIS 7235:2003 or the like).
The International Standard 1 EC60529 rule defines the Ingress Protection index with reference to some testing conditions, which may be more or less stringent, in which the electronic component shell is subjected to an intrusion of solid articles or water.
The first digit of the IP index is related to the solid material intrusion resistance.
Levels from IP1X to IP4X are usually of small interest for acoustic components, which, on the other hand, nearly always require the IP5X level, assuring a partial protection against a powder intrusion.
The requirement of an IP6X level, related to perfectly sealed or tightness components, on the contrary, is not common.
The second digit of the IP index refers to the water resistance, the IPX3, IPX4 and IPX5 levels showing a different amount of water spray resistance.
Usually, for the most common articles, such as cellular phones, the IPX3 level would be sufficient.
On the contrary, the “heavy duty” acoustic product market requires a protective level up to IPX8, corresponding to a water immersion resistance up to a depth of 10 meters for a time period up to 24 hours, which are very stringent conditions for the intended applications.
As stated, for a number of hand-held systems it is necessary to reduce the magnetic field/magnetic induction intensity at a given distance from the magnetic field source (that is the acoustic component) to prevent interferences with electronic components nearby, such as, for example, antennas, and to reduce the attraction exerted by said acoustic component on metal particles negatively affecting its operation.
Because of a designing trend to continuously reduce the electronic device thickness and size, the above magnetic field screening requirement will be very important to enhance the acoustic component protecting level and to reduce electromagnetic interferences with components arranged nearby.
To assess the screening efficiency of an acoustic component “protecting” means, no reference rule is known and, usually, this is achieved by:                directly testing the screen in its final configuration and verifying the absence of electromagnetic interferences with other components, if no magnetic field/magnetic induction maximum threshold value exists (at a set distance from the magnetic field source);        directly testing the screen in its final configuration and performing a magnetic induction value measurement at the outer opening, by arranging the magnetic field detecting probe in contact with the device casing;        verifying that the magnetic field/magnetic induction value is lower than the target threshold value;        measuring the magnetic field/magnetic induction value, either with or without the screen, in those same geometric conditions, to calculate the screening efficiency of the medium used.        
As above disclosed, at present different technical solutions are used, based on different textile products (non-woven, synthetic monofilament woven technical materials, water-repellent membranes), providing the acoustic and protecting performance required by modern acoustic products.
With respect to the acoustic, mechanical strength, processing and geometric coherency features, in case of a protecting requirement against particles of >15 μm size, and a IPX4 class water-repellent standard, the subject synthetic single-thread or monofilament technical fabrics provide the most suitable solution.
If said fabrics must also have a magnetic field screening capability, then are conventionally used small perforated plate elements of a ferromagnetic material (such as, for example, Mumetal, Permalloy, Metglas, Nanoperm, ferritic materials, ferromagnetic alloy steels or high relative magnetic permeability materials) arranged between the speaker and protective fabric, which plate elements are made by shearing sheet metal elements, which are further perforated by cold punching or chip removing operations.
It should be pointed out that, independently from the perforating method, no perforated ferromagnetic material rolls are at present available on the market, but only plate or sheet elements of a set size, which, in this prior art, are not coupled to other roll form media.
In fact, a coupling of plates having a set size would not be advantageous compared with a coupling of a roll form material.
The punched sheet metal family comprises those sheet metal elements which are perforated by perforating molds, including a punch and die assembly, mounted on punching or perforating press machines.
Thus, through the above punching, it is possible to provide several hole types fitted to different application requirements in any industrial fields. A drawback of the above perforating type is a hole diameter/sheet metal thickness ratio which, for a stainless steel (the most inexpensive material among those having a high magnetic permeability), cannot be less than 1.
The minimum thickness of steel plates perforated by a punching perforating method is typically of 0.3 mm.
Thus, considering the typical hole pitch values, from commercially available products having a thickness of 0.3 mm and a hole diameter of 0.3 mm, ferromagnetic steel perforated plates with a 1.84-2.13 kg/dm3 weight and a free surface from 10% to 22% are provided.
A number of applications do not require the above large amount of ferromagnetic material to provide the required screening efficiency, and the free surface thereof is a comparatively small one, which negatively affects the acoustic impedance.
To reduce the hole diameters (for a set plate thickness), plates perforated by multi-mandrel perforating heads are used, thereby it is possible to make holes of a size lower than the sheet metal element thickness while combining high empty/full rates with small passage cross-sections.
In such a case, the machining costs are much more higher, thereby this method is not suitable for small thickness (few tens of mm) plates and the product thus made is still a plate.
The prior art shows the drawbacks related to a use of the above perforated small plates:                a high cost of said perforated plates, due to the material perforating/shearing processes;        a difficult coupling of fabric rolls to perforated material rolls (plates);        a complex assembling of multiple protective layers on the acoustical component (for example the technical fabric plus a perforated plate or vice versa);        a need of increasing the number of components required for the assembling (gasket/adhesive tape) based on the number of protective layers used;        an increasing of the overall thickness because of an overlapping of two protective layers, plus the individual components required for assembling purposes, as schematically shown in FIG. 4;        an increasing of the device weight; and        an increasing of the acoustic impedance.        
The subject monofilament technical fabric material is per se very strong, tenacious, of even thickness, and it may be cut by the most common cutting methods in this field.
However, even if, from an acoustic standpoint, the square mesh synthetic fabric has very good acoustic properties, it still does not provide an efficient magnetic screening.
Document US 2010/304796 A1 discloses substantially the preamble of the main claim.
More specifically, document US 2010/304796 A1 discloses an electronic circuit carrier having at least a first and a second side, said sides facing away from each other, said circuit carrier comprising: a speaker placed on a first side arranged with a first magnetic shielding plate, and a second magnetic shielding plate arranged adjacent said second side. At least one of the magnetic shielding plates comprises one or more openings allowing passage of the sound produced by said loudspeaker. The magnetic shielding plates are made of a metal or of a metal alloy.
Document EP 0 609 223 B1 discloses an electromagnetic interference shielding filter comprising a porous composite material, allowing passage of gases, especially air, and comprising interconnected pores and voids that form passageways extending through the thickness and opening on both sides of said material, said material having electromagnetic radiation shielding properties comprising a layer of porous electrically conductive material (2), adhered to a layer of a porous material (15) coated with an amorphous copolymer of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole.
Document US 2013/032285 A1 discloses a laminar textile construction to be used in acoustic components, wherein said construction comprises a double layer arrangement made by coupling a technical synthetic single-thread fabric material to a polymeric film, for use as sub-components in acoustic and electric products in general.